The invention relates to a multi-tray apparatus for performing multistage countercurrent liquid-liquid extraction in a vertical column. The invention more specifically relates to a multi-tray apparatus which is mounted as a unit within a small diameter extraction column.
Liquid-liquid extraction is a commercially practiced process used to separate one component or class of components from a mixture. The liquid-phase feed stream is contacted with a liquid-phase solvent which is not miscible with the feed stream but which has a high capacity for dissolving the desired components of the feed stream. The success of an individual extraction step is often limited by many factors which hinder mass transfer and the quick establishment of equilibrium between the two liquid phases which are being brought into contact. Often several sequential extractions are required to reach a target recovery or purity. For these reasons many liquid-liquid extraction processes are performed using multiple contacting and separation steps performed in sequence with countercurrent flow of the two liquid streams. This can be done in vertical extraction columns through which the two liquids flow in opposite directions, with one phase flowing through openings in an active area of horizontal trays while the other phase flows horizontally past the openings of the contacting stages. The phase which flows horizontally across the tray then moves vertically to the next tray and stage of contacting, typically by means of enclosed conduit or channels.
U.S. Pat. No. 5,500,116 issued to T. Nakayama et al. illustrates in FIGS. 11 and 12 a conventional countercurrent flow system used in a liquid-liquid extraction column. In this flow system the xe2x80x9clightxe2x80x9d phase flows upward through the trays as the discontinuous phase and the xe2x80x9cheavyxe2x80x9d phase flows upward through channels and horizontally across the tray.
U.S. Pat. No. 4,247,521 issued to P. Forte describes several liquid-liquid tray designs and describes their operation. This reference shows several forms of xe2x80x9cupcomersxe2x80x9d for carrying the less dense liquid above the active area of the tray.
Cartridge trays are used in small diameter fractional distillation columns fabricated using large diameter pipe as the outer wall of the vessel. One or more bundles of trays are lowered into a vertical pipe to produce the column. As the edge of the tray is not attached to the wall of the pipe, a movable or flexible seal is usually provided at the edge of the tray. The construction of several forms of commercially available xe2x80x9cpackage traysxe2x80x9d is illustrated in a sales bulletin PT-3 published by the Nutter Engineering unit of the Harsco Corporation. This reference illustrates the circular trays fastened into unitary bundles and also shows the provision of seal rings at the edge of the trays.
The construction and operation of package trays is also described in U.S. Pat. No. 3,179,389 issued to I. E. Nutter. This specific reference is directed to vapor-liquid contacting trays such as used in fractional distillation. FIGS. 1 and 14 do however show multiple horizontal trays 36 held in position by four support rods. Downcomers 40 guide the descending liquid phase. This reference illustrates the use of a seal ring 46 at the periphery of the tray.
The invention is an improved design for a liquid-liquid extraction tray which finds application in cartridge type columns. The subject tray includes a partially circular wall which encircles the active area of the tray to lessen any leakage of dense phase liquid around the seal at the edge of the tray.
A broad embodiment of the invention may be characterized as an apparatus for installation into a vertical column used in a liquid-liquid extraction process, which apparatus comprises a plurality of discoid trays adapted to fit within the column in a horizontal plane with each tray comprising a seal ring, which engages the inner surface of the column, a perforated central active area, a chordal riser conduit on one side of the tray and a vertical wall surrounding the active area of the tray at a predetermined distance inward from the periphery of the tray.